A technique using a MIMO has become popular in the field of wireless communication, and the MIMO itself is becoming no longer a new technology. However, conventional techniques using the MIMO mainly focus on a mobile communication, and application of the MIMO to a fixed point communication has not been fully examined. In a mobile communication radio channels, radio wave coming from a transmission antenna is reflected or scattered according to the surrounding terrain and reaches a receiver in the form of a group of waves, resulting in occurrence of fading phenomenon which has been an obstacle to achievement of high quality communication. The MIMO technique in a mobile communication does not demonize the fading phenomenon but considers it as environmental resources with great potential that are inherent in mobile communication radio propagation. In this point, the MIMO technique is regarded as a revolutionary technique.
Although smaller in the amount of examples than the mobile communication, NPL (non-patent literature) 1 discloses consequents of application of such a MIMO technique to a line-of-sight fixed point radio communication where radio channels are determined. The mobile communication as described above deals with channels as a random matrix. On the other hand, the line-of-sight fixed point radio communication needs to deal with channels as deterministic channels. The above NPL 1 describes, as follows, what effect is produced on a channel matrix H constituting channels between transmission and reception antennas as a result of extension of antenna interval on both the transmission side and reception side.H·HH=n·In  [Numeral 1]where n is the number of antennas, HH is the Hermitian transposed matrix of channel matrix H, and I is a unit matrix, and the phase rotation of a signal with respect to a transmission antenna i and reception antenna k linearly arranged so as to face each other between the transmission side and reception side is set by the following formula and thereby the transmission and reception antennas can be constituted by linear antennas.
                              π          n                ·                              [                          i              -              k                        ]                    2                                    [                  Numeral          ⁢                                          ⁢          2                ]            
Assuming that n=2, the channel matrix H is represented by the following formula.
                              H          max                =                  [                                                    1                                            j                                                                    j                                            1                                              ]                                    [                  Numeral          ⁢                                          ⁢          3                ]            
In this case, an antenna configuration satisfying the condition of Numeral 1 is possible. NPL 1 describes that when the condition of Numeral 1 is satisfied, channel capacity in the MIMO configuration becomes maximum by Hmax. That is, an increase in channel capacity based on the MIMO can be achieved not only in a mobile communication environment that is subject to reflection or scattering but also in a deterministic line-of-sight communication environment.
Now, considering a case where such a deterministic line-of-sight MIMO is applied to a small fixed point microwave communication system. In general, the small fixed point microwave communication system uses a frequency band of several GHz to several tens of GHz, which corresponds to several mm to several cm in terms of wavelength. Therefore, a significant phase rotation may occur due to movement in the antenna direction highly sensitive to a subtle change of weather condition such as wind or surrounding temperature. Under such a condition, it is difficult to ensure the deterministic channel matrix. Note that theoretical analysis to be described later analytically reveals that the above increase in channel capacity can be achieved even when such a displacement in the highly sensitive antenna direction occurs.
In the MIMO technique, a plurality of independent signals are transmitted/received at the same frequency band. Therefore, signal separation/detection is necessary. As a means for realizing this, there is a known a method (hereinafter, referred to as SVD method) based on matrix calculation using a unitary matrix which is obtained by Singular Value Decomposition (SVD). Assume that feedback information for construction of the unitary matrix can ideally be send from a reception end to transmission end in the SVD method. In this case, even if the above displacement in the highly sensitive antenna direction occurs, the unitary matrix acts so as to compensate for the displacement. As a result, large capacity fixed point microwave communication can be realized based on the MIMO. However, the above feedback information may increase system overhead. In addition, it is necessary to prepare a reverse channel for exchanging the feedback information. Note that a modeling of a channel matrix H to be described later performs analysis including the displacement in the highly sensitive antenna direction.
When the singular value analysis is carried out for the line-of-sight fixed channels where channels are deterministic, there exists an inter-antenna position at which an eigenvalue is a multiplicity condition to generate a singular point. Although the singular-value is uniquely determined, singular vectors are not unique. This state, which is particularly analytically troublesome, may cause significant transition of the singular vectors. However, by utilizing this phenomenon, various configurations can be possible. Various examples of configurations that take advantage of the characteristics will be described in detail later.
As a major problem in the deterministic line-of-sight MIMO, there is a problem that carrier synchronization between antennas must be achieved on the transmission side or reception side in the above conventional method. That is, the phase between a plurality of antennas on the transmission side or reception side needs to be equal or needs to have a constant phase difference.
On the other hand, in the fixed point microwave communication system, antenna interval must be widened in view of a frequency to be used. Correspondingly, radio devices including local oscillators are installed near antennas. That is, the problem of the necessity of achievement of carrier synchronization between antennas imposes severe restriction on construction of the fixed point microwave communication system.